In recent years, from a viewpoint of preventing air pollution, an ammonia catalytic reduction method in which ammonia is used as a reducing agent to catalytically decompose nitrogen oxides (hereinafter, referred to as NOx) into nitrogen and water by a catalyst has been widely used as a method of removing NOx produced from a boiler or various combustion furnaces. As the NOx removal catalyst, which has been currently put into practical use, a honeycomb catalyst having a rectangular hole shape has been the mainstream, in order to prevent blockages due to dust in flue gas and to widen a gas contact area. Furthermore, as a catalyst component, a type of catalyst component containing titanium oxide as a main component is excellent, and one containing vanadium, tungsten or the like as an active component is generally used, and a binary TiO2-WO3 catalyst or TiO2-MoO3 catalyst, a ternary TiO2-V2O5-WO3 catalyst or TiO2-V2O5-MoO3 catalyst and the like are mainly used.
In flue gas discharged from a coal combustion boiler, a calcium content contained in dust in flue gas mainly adheres to a surface of the catalyst to produce anhydrous calcium sulfate contained in flue gas and cover the surface of the catalyst, thereby blocking diffusion of NO and NH3 gas to the inside of the catalyst and deteriorating the performance of the catalyst. Conventionally, at the time of regeneration of a catalyst, whose performance is deteriorated due to these causes of deterioration, washing with water and an aqueous solution of hydrochloric acid has been known to be effective.
Furthermore, there has been proposed a technique for performing activation treatment of a catalyst with an acidic aqueous solution after removing a substance accumulated on the catalyst by washing it with an aqueous alkaline solution, at the time of the regeneration of a NOx removal catalyst whose NOx removal performance deteriorates due to accumulation of arsenic (As2O3) present in flue gas, (Patent Literature 1).